Can I stain a darker stain over lighter stain?

Yes you can. Use Minwax Polyshades.

http://www.minwax.com/how-to-finish-wood/change-stain-color-with-polyshades/

You would have to take whatever finish is on top off first. Once that is gone, you can add a darker stain; however the existing colour will change the colour of the new stain....I often use two or three different stains to get the colour/effect I am looking for

Yes but the tone of the stain may influence the end color, especially if its a reddish stain or a real strong gold color. Try in an area that you don't see too much to test.

Yes you can. If you remove the finish, and just have stained bare wood, you can pit another stain on top of it. However, the resulting color may be affected by the current stain color.

If the finish is still on the current piece, you can go over it with a product that is both stain and finish. Give the current finish a light sanding. You may need several coats to get the color you want.

yes but you need to lightly sand first

Sanding will not remove the stain. Stain gets absorbed into the wood, You would never get it all out.